<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        ul {
            list-style: none;
            margin: 0;
            padding: 0;
        }

        .box {
            width: 500px;
            height: 400px;
            margin:0 auto;
            background-color: #ccc;
            display: flex;
            flex-wrap: wrap;
            justify-content: space-around;
            align-content: space-around;
        }

        .box li {
            width: 150px;
            height: 150px;
            background-color: #999;
        }


    </style>
</head>
<body>
    <ul class="box">
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
    </ul>
</body>
</html>